1. Definition and Measurement of Gear Surface Roughness

1.1 Key Evaluation Parameters

• Ra (Arithmetic Mean Deviation): The average of absolute deviations of profile points from the reference line, serving as the most commonly adopted roughness indicator.
• Rz (Maximum Height of Roughness Profile): The maximum vertical distance between the peak line and valley line of the profile.
• Rq (Root Mean Square Deviation): The root mean square value of profile deviations, which better reflects the impact of extreme peaks and valleys compared to Ra.

1.2 Common Measurement Methods

• Contact Measurement (Profilometer): Utilizes a diamond stylus to scan the surface, offering high precision but posing a risk of scratching soft material surfaces.
• Non-Contact Measurement (White Light Interferometer, Laser Confocal Microscope): Suitable for high-precision and ultra-smooth surface testing without causing contact damage.
• Comparison Specimen Method (Ra Template Comparison): Enables on-site rapid inspection with relatively lower precision.

2. Design Principles of Gear Surface Roughness

2.1 Roughness Grade Classification (ISO 1328 & AGMA 2015 Standards)

2.2 Impact of Roughness on Gear Performance

• Friction and Lubrication: Excessively high roughness makes it difficult to form an oil film, leading to boundary lubrication or even dry friction and increasing wear risks. Conversely, extremely low roughness reduces lubricant adsorption, which may impair lubrication effects (e.g., certain polymer gears require a specific roughness to retain oil).
• Contact Fatigue Life: Microscopic peaks and valleys (resulting from roughness) are prone to stress concentration under contact stress, accelerating pitting and spalling. Optimizing Ra (e.g., Ra=0.2-0.4μm for most industrial gears) can effectively improve contact fatigue life.
• Vibration and Noise: Rough tooth flanks cause meshing impact, increasing transmission noise (e.g., automotive transmission gears typically require Ra ≤ 0.4μm).
• Initial Running-in Characteristics: Appropriate roughness (e.g., Ra=0.6-1.0μm) facilitates initial running-in, allowing tooth flanks to quickly adapt to load distribution.

3. Influencing Factors and Engineering Applications

3.1 Impact of Machining Processes

• Grinding: Produces Ra values of 0.2-0.8μm, suitable for high-precision gears.
• Hobbing/Shaping: Results in Ra values of 0.8-1.6μm, applicable to general industrial gears.
• Honing/Lapping: Achieves Ra ≤ 0.2μm, used for ultra-precision gears (e.g., aero-engine gears).
• Shot Peening: Improves surface roughness distribution and enhances fatigue resistance.

3.2 Effect of Material and Heat Treatment

• Hardened Gears (Carburizing and Quenching): After grinding, Ra is usually controlled below 0.4μm.
• Soft Gears (Tempering Treatment): Permit higher roughness (Ra=0.8-1.6μm), but initial running-in must be considered.

3.3 Influence of Lubrication Conditions

• Mineral Oil Lubrication: Ra is recommended to be ≤ 0.8μm.
• Synthetic Oil/Extreme Pressure Lubrication: Can tolerate higher roughness (e.g., Ra=1.0-1.6μm).
• Dry Friction/Self-Lubricating Gears (e.g., Engineering Plastics): Require a specific roughness (Ra=1.0-2.0μm) to store solid lubricants.

3.4 Typical Engineering Application Cases

• Automotive Transmission Gears (High-Speed, Low-Noise): Ra=0.2-0.4μm (grinding + honing). Superfinishing is adopted to reduce vibration and noise (e.g., Ra ≤ 0.2μm for electric vehicle reducer gears).
• Wind Turbine Gearboxes (Heavy-Duty, Long-Life): Ra=0.4-0.8μm (grinding + shot peening). Roughness distribution is optimized to reduce pitting risks.
• Construction Machinery Gears (Low-Speed, High-Impact): Ra=0.8-1.6μm (hobbing + phosphating). Appropriate roughness is retained to improve running-in performance.

3.5 Surface Treatment Technologies for Roughness Optimization

• Lapping/Polishing: Further reduces Ra, suitable for precision gears.
• Coating Technology (e.g., DLC Diamond-Like Carbon Coating): Lowers friction coefficient and adapts to high-roughness working conditions.
• Laser Microtexturing: Processes micro-pits or grooves on tooth flanks to optimize lubricating film distribution.

4. Summary